The present invention relates to the field of touch sensors, including touch screens and touch pads, and their associated control chip(s). In particular, embodiments of the invention relate to designs for electrode patterns for such sensors for sensing the presence of one or more touching objects in a sensing area arranged across a three-dimensional surface
A capacitive touch sensor can be generalised as one that uses a physical sensor element comprising an arrangement of electrically conductive electrodes extending over a touch sensitive area (sensing area) and a controller chip connected to the electrodes and operable to measure changes in the electrical capacitance of each of the electrodes or the mutual-capacitance between combinations of the electrodes.
FIG. 1 schematically shows principal components of a generic conventional capacitive touchscreen comprising a physical sensor element 100. The touch screen is represented in plan view (to the left in the figure) and also in cross-sectional view (to the right in the figure).
The touch screen is configured for establishing the position of a touch within a two-dimensional sensing area by providing Cartesian coordinates along an X-direction (horizontal in the figure) and a Y-direction (vertical in the figure). In this example the sensor element 100 is constructed from a substrate 103 that could be glass or plastic or some other insulating material and upon which is arranged an array of electrodes consisting of multiple laterally extending parallel electrodes, X-electrodes 101, and multiple vertically extending parallel electrodes, Y-electrodes 102, which in combination allow the position of a touch 109 to be determined. To clarify the terminology, and as will be seen from FIG. 1, the X-electrodes 101 are aligned parallel to the X-direction and the Y-electrodes 102 are aligned parallel to the Y-direction. Thus the different X-electrodes allow the position of a touch to be determined at different positions along the Y-direction while the different Y-electrodes allow the position of a touch to be determined at different positions along the X-direction. That is to say in accordance with the terminology used herein, the electrodes are named after their direction of extent rather than the direction along which they resolve position.
In some cases, each electrode may have a more detailed structure than the simple “bar” structures represented in FIG. 1, but the operating principles are broadly the same. The electrodes, which are sometimes called traces, are made of an electrically conductive material such as copper or Indium Tin Oxide (ITO). The nature of the various materials used depends on the desired characteristics of the touch screen. For example, a touch screen may need to be transparent, in which case ITO electrodes and a plastic substrate are common. On the other hand a touch pad, such as often provided in lieu of a mouse in laptop computers is usually opaque, and hence can use lower cost copper electrodes and an epoxy-glass-fibre substrate (e.g. FR4). Referring back to the figure, the electrodes are electrically connected via circuit conductors 104 to a controller chip 105, which is in turn connected to a host processing system 106 by means of a communication interface 107. The host 106 interrogates the controller chip 105 to recover the presence and coordinates of any touch or touches present on, or proximate to the sensor 103. In the example, a front cover (also referred to as a lens or panel) 108 is positioned in front of the sensor 103 and a single touch 109 on the surface of the cover 108 is schematically represented.
Note that the touch itself does not generally make direct galvanic connection to the sensor 103 or to the electrodes 102. Rather, the touch influences the electric fields 110 that the controller chip 105 generates using the electrodes 102. With appropriate analysis of relative changes in the electrodes' measured capacitance/capacitive coupling, the controller chip 105 can thus calculate a touch position on the cover's surface as an XY coordinate 111. The host system can therefore use the controller chip to detect where a user is touching, and hence take appropriate action, perhaps displaying a menu or activating some function.
There are many different material combinations and electrode configurations to allow creation of a touch screen and the example discussed above is just one.
A further aspect of capacitive touch sensors relates to the way the controller chip uses the electrodes of the sensor element to make its measurements. There are two main classes of controller in this regard.
A first class is known as a “self-capacitance” style. Reference is made to FIG. 2. In this design of a capacitive sensor, the controller 201 will typically apply some electrical stimulus (drive signal) 202 to each electrode 203 which will cause an electric field to form around it 204. This field couples through the space around the electrode back to the controller chip via numerous conductive return paths that are part of the nearby circuitry 205, product housing 206, physical elements from the nearby surroundings 207 etc., so completing a capacitive circuit 209. The overall sum of return paths is typically referred to as the “free space return path” in an attempt to simplify an otherwise hard-to-visualize electric field distribution. The important point to realise is that the controller is only driving each electrode from a single explicit electrical terminal 208; the other terminal is the capacitive connection via this “free space return path”. The capacitance measured by the controller is the “self-capacitance” of the electrode (and connected tracks) relative to free space (or Earth as it is sometimes called) i.e. the “self-capacitance” of the electrode. Touching or approaching the electrode with a conductive element 210, such as a human finger, causes some of the field to couple via the finger through the connected body 213, through free space and back to the controller. This extra return path 211 can be relatively strong for large objects (such as the human body), and so can give a stronger coupling of the electrode's field back to the controller; touching or approaching the electrode hence increases the self-capacitance of the electrode. The controller is configured to sense this increase in capacitance. The increase is strongly proportional to the area 212 of the applied touch and is normally weakly proportional to the touching body's size (the latter typically offering quite a strong coupling and therefore not being the dominant term in the sum of series connected capacitances).
In a classic self-capacitance sensor the electrodes are arranged on an orthogonal grid, generally with a first set of electrodes on one side of a substantially insulating substrate and the other set of electrodes on the opposite side of the substrate and oriented at nominally 90° to the first set. There are also structures where the grid is formed on a single side of the substrate and small conductive bridges are used to allow the two orthogonal sets of electrodes to cross each other without short circuiting. One set of electrodes is used to sense touch position in a first axis that we shall call “X” and the second set to sense the touch position in the second orthogonal axis that we shall call “Y”.
In a self-capacitance touch sensor, the controller can either drive each electrode in turn (sequential) or it can drive them all in parallel. In the former sequential case, any neighbouring electrodes to a driven electrode are sometimes grounded by the controller to prevent them becoming touch sensitive when they are not being sensed (remembering that all nearby capacitive return paths will influence the measured value of the actively driven electrode). In the case of the parallel drive scheme, the nature of the stimulus applied to all the electrodes is typically the same so that the instantaneous voltage on each electrode is approximately the same. The drive to each electrode is electrically separate so that the controller can discriminate changes on each electrode individually, but the driving stimulus in terms of voltage or current versus time, is the same. In this way, each electrode has minimal influence on its neighbours (the electrode-to-electrode capacitance is non-zero but its influence is only “felt” by the controller if there is a voltage difference between the electrodes).
The second class of controller is known as a “mutual-capacitance” style. Reference is made to FIG. 3. In this design of a capacitive sensor the controller 301 will sequentially stimulate each of an array of transmitter (driven/drive) electrodes 302 that are coupled by virtue of their proximity to an array of receiver electrodes 303. The resulting electric field 304 is now directly coupled from the transmitter to each of the nearby receiver electrodes; the “free space” return path discussed above plays a negligible part in the overall coupling back to the controller chip when the sensor is not being touched. The area local to and centred on the intersection of a transmitter and a receiver electrode is typically referred to as a “node”. Now, on application or approach of a conductive element 305 such as a human finger, the electric field 304 is partly diverted to the touching object 305. An extra return path to the controller 301 is now established via the body 306 and “free-space” in a similar manner to that described above. However, because this extra return path acts to couple the diverted field directly to the controller chip 301, the amount of field coupled to the nearby receiver electrode 303 decreases. This is measured by the controller chip 301 as a decrease in the “mutual-capacitance” between that particular transmitter electrode and receiver electrodes in the vicinity of the touch. The controller senses this change in capacitance of one or more nodes. The magnitude of a capacitance change is nominally proportional to the area 307 of the touch (although the change in capacitance does tend to saturate as the touch area increases beyond a certain size to completely cover the nodes directly under the touch) and weakly proportional to the size of the touching body (for reasons as described above). The magnitude of the capacitance change also reduces as the distance between the touch sensor electrodes and the touching object increases.
In a classic mutual-capacitance sensor the transmitter electrodes and receiver electrodes are arranged as an orthogonal grid, with the transmitter electrodes on one side of a substantially insulating substrate and the receiver electrodes on the opposite side of the substrate. This is as schematically shown in FIG. 3. It should be understood that discussion of a single unitary substrate does not preclude use of a multi-layer substrate which can sometimes be advantageous for cost, ease of fabrication or for other reasons. In FIG. 3 a first set of transmitter electrodes 303 is shown on one side of a substantially insulating substrate 308 and a second set of receiver electrodes 302 is arranged at nominally 90° to the transmitter electrodes on the other side of the substrate. There are also structures where the grid is formed on a single side of the substrate and small insulating bridges are used to allow the transmitter and receiver electrodes to cross each other without short circuiting.
By using interpolation between adjacent nodes for both types of capacitive touch sensor a controller chip can typically determine touch positions to a greater resolution than the spacing between electrodes. Also there are established techniques by which multiple touches within a sensing area, and which might be moving, can be uniquely identified and tracked, for example until they leave the sensing area.
It will be appreciated that conventional position-sensitive touch sensors are generally configured to provide a position measurements as a Cartesian coordinate within a two-dimensional sensing space which is defined relative to the X- and Y-electrodes. Accordingly, commercially-available capacitive sensing controller chips (ICs) are generally designed to interface to linear arrays of straight sensor electrodes, or X and Y electrodes, in a flat, two-dimensional plane.
In some cases a conventional X-Y grid electrode layout coupled to a conventional controller may not be desired for a particular implementation. This might be, for example because a more complex sensing surface is desired, or because of restrictions on the way in which physical connections can easily be made between a controller and sensor electrodes.
With the above issues in mind there is a need for alternative sensor designs providing for more flexibility in respect of different shapes of sensing surface and connectability between sensor electrodes and control circuitry.